Re: [PATCH 1/2 v4] mm: vmscan: do not pass reclaimed slab to vmpressure

[vinayak menon]

On Mon, Feb 6, 2017 at 6:22 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Mon 06-02-17 17:54:09, Vinayak Menon wrote:
>> During global reclaim, the nr_reclaimed passed to vmpressure includes the
>> pages reclaimed from slab.  But the corresponding scanned slab pages is
>> not passed.  This can cause total reclaimed pages to be greater than
>> scanned, causing an unsigned underflow in vmpressure resulting in a
>> critical event being sent to root cgroup.
>
> If you switched the ordering then this wouldn't be a problem, right?
You mean calling vmpressure first and then shrink_slab ? That also can be
done. Would completing shrink_slab before vmpressure be of any use to a
userspace task that takes into account both vmpressure and reclaimable
slab size ?
>
>> It was also noticed that, apart
>> from the underflow, there is an impact to the vmpressure values because of
>> this. While moving from kernel version 3.18 to 4.4, a difference is seen
>> in the vmpressure values for the same workload resulting in a different
>> behaviour of the vmpressure consumer. One such case is of a vmpressure
>> based lowmemorykiller. It is observed that the vmpressure events are
>> received late and less in number resulting in tasks not being killed at
>> the right time. The following numbers show the impact on reclaim activity
>> due to the change in behaviour of lowmemorykiller on a 4GB device. The test
>> launches a number of apps in sequence and repeats it multiple times.
>>                       v4.4           v3.18
>> pgpgin                163016456      145617236
>> pgpgout               4366220        4188004
>> workingset_refault    29857868       26781854
>> workingset_activate   6293946        5634625
>> pswpin                1327601        1133912
>> pswpout               3593842        3229602
>> pgalloc_dma           99520618       94402970
>> pgalloc_normal        104046854      98124798
>> pgfree                203772640      192600737
>> pgmajfault            2126962        1851836
>> pgsteal_kswapd_dma    19732899       18039462
>> pgsteal_kswapd_normal 19945336       17977706
>> pgsteal_direct_dma    206757         131376
>> pgsteal_direct_normal 236783         138247
>> pageoutrun            116622         108370
>> allocstall            7220           4684
>> compact_stall         931            856
>
> From this numbers it seems that the memory pressure was higher in 4.4.
> There is ~5% more allocations in 4.4 while we hit the direct reclaim 50%
> more times.
When the fix is applied to 4.4 the above reclaim stat looks almost similar
for 4.4 and 3.18. That means the memory pressure seen is a side effect of the
problem that the patch addresses.

>
> But the above doesn't say anything about the number and levels of
> vmpressure events. Without that it is hard to draw any conclusion here.
>
For this usecase, the number of critical vmpressure events received is around
50% less on 4.4 base than 3.18.

> It would be also more than useful to say how much the slab reclaim
> really contributed.
The 70% less events is caused by slab reclaim being added to vmpressure,
which is confirmed by running the test with and without the fix.
But it is hard to say the effect on reclaim stats is caused by this
problem because,
the lowmemorykiller can be written with different heuristics to make the reclaim
look better. The issue we see in the above reclaim stats is entirely
because of task kills
being delayed. That is the reason why I did not include the vmstat stats in the
changelog in the earlier versions.

>
>> This is a regression introduced by commit 6b4f7799c6a5 ("mm: vmscan:
>> invoke slab shrinkers from shrink_zone()").
>
> I am not really sure this is a regression, though. Maybe your heuristic
> which consumes events is just too fragile?
>
Yes it could be. A different kind of lowmemorykiller may not show up this issue
at all. In my opinion the regression here is the difference in vmpressure values
and thus the vmpressure events because of passing slab reclaimed pages
to vmpressure without considering the scanned pages and cost model.
So would it be better to drop the vmstat data from changelog ?

>> So do not consider reclaimed slab pages for vmpressure calculation. The
>> reclaimed pages from slab can be excluded because the freeing of a page
>> by slab shrinking depends on each slab's object population, making the
>> cost model (i.e. scan:free) different from that of LRU.  Also, not every
>> shrinker accounts the pages it reclaims.
>
> Yeah, this is really messy and not 100% correct. The reclaim cost model
> for slab is completely different to the reclaim but the concern here is
> that we can trigger higher vmpressure levels even though there _is_ a
> reclaim progress. This should be at least mentioned in the changelog so
> that people know that this aspect has been considered.
>
Yes I can add that.

>> Fixes: 6b4f7799c6a5 ("mm: vmscan: invoke slab shrinkers from shrink_zone()")
>> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
>> Cc: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
>> Cc: Shiraz Hashim <shashim@xxxxxxxxxxxxxx>
>> Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
>> ---
>>  mm/vmscan.c | 17 ++++++++++++-----
>>  1 file changed, 12 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 947ab6f..8969f8e 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2594,16 +2594,23 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>>                                   sc->nr_scanned - nr_scanned,
>>                                   node_lru_pages);
>>
>> +             /*
>> +              * Record the subtree's reclaim efficiency. The reclaimed
>> +              * pages from slab is excluded here because the corresponding
>> +              * scanned pages is not accounted. Moreover, freeing a page
>> +              * by slab shrinking depends on each slab's object population,
>> +              * making the cost model (i.e. scan:free) different from that
>> +              * of LRU.
>> +              */
>> +             vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
>> +                        sc->nr_scanned - nr_scanned,
>> +                        sc->nr_reclaimed - nr_reclaimed);
>> +
>>               if (reclaim_state) {
>>                       sc->nr_reclaimed += reclaim_state->reclaimed_slab;
>>                       reclaim_state->reclaimed_slab = 0;
>>               }
>>
>> -             /* Record the subtree's reclaim efficiency */
>> -             vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
>> -                        sc->nr_scanned - nr_scanned,
>> -                        sc->nr_reclaimed - nr_reclaimed);
>> -
>>               if (sc->nr_reclaimed - nr_reclaimed)
>>                       reclaimable = true;
>>
>> --
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
>> member of the Code Aurora Forum, hosted by The Linux Foundation
>>
>
> --
> Michal Hocko
> SUSE Labs
>
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